The present invention generally relates to the field of implantable medical devices intended to maintain patency of anatomical passageways, such as those found in the cardiovascular, lymphatic, endocrine, renal, gastrointestinal and/or reproductive systems of mammals. More particularly, the present invention relates to grafts that are designed for endoluminal delivery using a delivery catheter and minimally invasive surgical techniques. The present invention generally comprises grafts or graft assemblies that are fabricated entirely of biocompatible metals or of biocompatible materials that exhibit biological response and material characteristics substantially the same as biocompatible metals, such as for example composite materials.
Conventional endoluminal stents and stent-grafts are frequently used after a percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA) procedure which dilitates an occluded, obstructed or diseased anatomical passageway to provide structural support and maintain the patency of the anatomical passageway. An example of this is the post-angioplasty use of intravascular stents to provide a structural support for a blood vessel and reduce the incidence of restenosis. A principal, but non-limiting, example of the present invention are endovascular stents which are introduced to a site of disease or trauma within the body's vasculature from an introductory location remote from the disease or trauma site using an introductory catheter, passed through the vasculature communicating between the remote introductory location and the disease or trauma site, and released from the introductory catheter at the disease or trauma site to maintain patency of the blood vessel at the site of disease or trauma. Stent-grafts are delivered and deployed under similar circumstances and are utilized to maintain patency of an anatomic passageway, for example, by reducing restenosis following angioplasty, or when used to exclude an aneurysm, such as in aortic aneurysm exclusion applications.
While the use of endoluminal stents has successfully decreased the rate of restenosis in angioplasty patients, it has been found that a significant restenosis rate continues to exist in spite of the use of endoluminal stents. It is generally believed that the post-stenting restenosis rate is due, in major part, to the non-regrowth of the endothelial layer over the stent and the incidence of smooth muscle cell-related neointimal growth on the luminal surfaces of the stent. Injury to the endothelium, the natural nonthrombogenic lining of the arterial lumen, is a significant factor contributing to restenosis at the situs of a stent. Endothelial loss exposes thrombogenic arterial wall proteins, which, along with the generally thrombogenic nature of many prosthetic materials, such as stainless steel, titanium, tantalum, Nitinol, etc. customarily used in manufacturing stents, initiates platelet deposition and activation of the coagulation cascade, which results in thrombus formation, ranging from partial covering of the luminal surface of the stent to an occlusive thrombus. Additionally, endothelial loss at the site of the stent has been implicated in the development of neointimal hyperplasia at the stent situs. Accordingly, rapid re-endothelialization of the arterial wall with concomitant endothelialization of the body fluid or blood contacting surfaces of the implanted device is considered critical for maintaining vasculature patency and preventing low-flow thrombosis.
At present, most endoluminal stents are manufactured of metals that fail to promote redevelopment of a healthy endothelium and/or are known to be thrombogenic. In order to increase the healing and promote endothelialization, while maintaining sufficient dimensional profiles for catheter delivery, most stents minimize the metal surface area that contacts blood. Thus, in order to reduce the thrombogenic response to stent implantation, as well as reduce the formation of neointimal hyperplasia, it would be advantageous to increase the rate at which endothelial cells form endothelium proximal and distal to the stent situs, migrate onto and provide endothelial coverage of the luminal surface of the stent which is in contact with blood flow through the vasculature.
Current stent-grafts are essentially endoluminal stents with a discrete covering on either or both of the luminal and abluminal surfaces of the stent that occludes the open spaces, or interstices, between adjacent structural members of the endoluminal stent. It is known in the art to fabricate stent-grafts by covering the stent with endogenous vein or a synthetic material, such as woven polyester known as DACRON, or with expanded polytetrafluoroethylene. Additionally, it is known in the art to cover the stent with a biological material, such as a xenograft or collagen. A primary purpose for covering stents with grafts is to reduce the thrombogenic effect of the stent material. However, the use of conventional graft materials has not proven to be a complete solution to enhancing the healing response of conventional stents.
U.S. Pat. No. 6,312,463 describes a variation of a prosthesis in that the prosthesis includes a tubular element that is a thin-walled sheet having temperature-activated shape memory properties. The tubular element is supported by a support element that includes a plurality of struts. The tubular element is described as a thin-walled sheet preferably having of a coiled-sheet configuration with overlapping inner and outer sections.
There still remains a need in the art for an implantable endoluminal graft that provides the necessary structural component to support an endoluminal wall and is biocompatible and prevents or limits the occurrence of restenosis.